transport: unify return values and exit point from transport_push()

transport: unify return values and exit point from transport_push()

It seems there is no reason to return 1 instead of -1 when push_refs()
is not set in transport vtable. Let's unify the error return values and
use the done label as a single exit point from transport_push().

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Frantisek Hrbata <frantisek@hrbata.com>
Reviewed-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>